Page 107 - Wastewater Solids Incineration Systems

76 Wastewater Solids Incineration Systems

• Flexibility, suitable to feed variability. Within a defined size range, the fluid
bed can easily handle feeds with varying chemical properties, moisture, and
volatile contents, particularly during short periods of time. This characteristic
is primarily a result of the large quantity of heat stored in the sand bed and to
the fact that the feed represents approximately only 1% of total bed material.
• Ease of control and automation. The rapid mixing of solids caused by the tur-
bulence in a fluid bed provides uniformity of bed temperature. Temperature
control is reduced to basically one point of control, which is the average of all
measurements.
• Lower auxiliary fuel usage. Much less fuel is required, mainly because of rela-
tively low excess air requirements (only 40% excess over stoichiometry com-
pared with typically 100% excess over stoichiometry in other thermal tech-
nologies) and possibility of heat recovery from flue gas and preheating of
combustion air to approximately 650°C (1200°F).
• Reduced maintenance costd. There is no moving part exposed to the combus-
tion section. Absence of thermal shock from the thermal reservoir of the bed
solids, results in slow temperature changes and leads to longer refractory life
and lower maintenance cost.

• High efficiency of combustion. High turbulences of the sand bed increase the
contact surface of cake particles and oxygen, increasing combustion efficiency.
Combustion is basically instantaneous, with low emissions of CO and THC.
• Low NOx emissions, primarily because of low excess air and good distribu-
tion of temperature throughout the bed.

9.0 MULTIPLE-HEARTH FURNACE
The MHF was developed in 1888 for roasting ores that contained metal sulfides to
isolate metals from further refining. Nichols Engineering—a New Jersey firm that
designed and built Hereshoff-type metallurgical furnaces in the early part of this cen-
tury—realized that the extended holding time and positive material flow the furnace
offered was well suited to the needs of countercurrent drying and burning waste-
water filter cake. This product of dewatering must be dried to approximately 50 to
55% moisture before it can sustain its own combustion; but it is sticky and rapidly
fouls indirect heating surfaces. It was found that the plowing action, called rabbling,